Color images are well known to be formed by reacting couplers with color developing agents of aromatic primary amine type which have been oxidized by optically exposed silver halides, which function as oxidizing agent to produce indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine and their analogous dyes.
Most of the magenta color image-forming couplers which have widely been submitted to practical use up to the present, studies of which have been proceeded, are 5-pyrazolones. The 5-pyrazolone couplers are excellent in dye-forming speed and efficiency and, moreover, the azomethine dyes produced therefrom are excellent in fastness to light and heat. However, it is also known that such azomethine dyes have absorption of a yellow component in the neighborhood of 430 nm, which is undesirable as a magenta dye and responsible for color turbidity.
As a result of seeking after magenta color image-forming coupler nuclei which enable reduction of this yellow component, pyrazolobenzimidazole nuclei described in British Patent 1,047,612, pyrazolotriazole nuclei described in U.S. Pat. No. 3,725,067, pyrazoloimidazole nuclei described in U.S. Pat. No. 4,500,630, pyrazolopyrazole nuclei described in Japanese Patent Application (OPI) No. 43659/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and pyrazolotriazole nuclei described in European Patent 119,860A have been developed.
All of the dyes formed from these nuclei are superior to those formed from the former 5-pyrazolones in respect of unnecessary absorption of the yellow component, and, further, they are desirable from the viewpoint of color reproducibility, for the absorption on the long wavelength side in their absorption spectra descends sharply to the zero level.
However, we have found that these pyrazoloazole couplers, though superior in the above-described respects, are possessed of undesirable properties as described below.
Namely, when these couplers are present together with silver halides to function as oxidizing agent for an aromatic primary amine developing agent, more specifically when emulsified dispersions of these couplers and silver halide emulsions are mixed, coated on supports respectively, and examined for photographic properties, the silver halide emulsions used cannot manifest their intrinsic sensitivity, gradation and fog. That is, some emulsions undergo sensitization, some emulsions undergo desensitization, and the others undergo decrease in color density of developed images. These behaviors can be ascertained, e.g., by comparing with the result of color development or black-and-white development conducted in the emulsion-coated sample in which a 5-pyrazolone type coupler is incorporated under the same condition as pyrazoloazole type couplers. Also, part of the above-described phenomena can be understood by preparing a coat which contains a silver halide emulsion alone and does not contain any couplers, and comparing the result of black-and-white development of the coat with that of a silver halide emulsion coat containing a pyrazoloazole type coupler.
In the nature of things, couplers to be employed in silver halide color photographic materials, exclusive of development inhibitor releasing couplers and development accelerator releasing couplers, are not expected to exert any actions on silver halide emulsions, and are not desired to produce a sensitization, desensitization or like effect by acting particularly on the sensitization process.
The above-described actions on silver halides were found to be hardly caused by 5-pyrazolone couplers which have prevailingly been put to practical use as magenta color image-forming coupler up to the present, whereas such actions have turned out to be extensively observed in case of pyrazoloazole couplers.
The pyrazoloazole couplers are couplers having a pyrazolobenzimidazole ring, a pyrazolotriazole ring or the like. We have confirmed that compounds containing residues of these rings have stronger interaction upon silver ion or silver halide, more specifically complexation power thereover or adsorptive activity thereon, compared with compounds containing residues of 5-pyrazolones which have widely used as magenta coupler up to the present. Further, it has also been ascertained that these interactions result in sensitization, desensitization or drop in color formability.
Variations of interaction between a silver halide emulsion and a pyrazoloazole coupler with the kind of halide contained in the emulsion were examined using samples prepared by coating combinations of various kinds of silver halide emulsions and a pyrazoloazole coupler on separate supports. As a result of these examinations, it has turned out that the pyrazoloazole coupler is more liable to cause the drop in color formability in a silver chloride emulsion than in a silver bromide emulsion. Moreover, it has been found that a silver iodobromide emulsion which contains silver iodide is more difficult to undergo the lowering of color formability than a silver bromide emulsion and a silver chlorobromide emulsion. That is, there was a greater interaction between a silver chlorobromide emulsion and a pyrazoloazole coupler than between a silver iodobromide emulsion and a pyrazoloazole coupler in regard to drop in color formability. Therefore, it was still more difficult to introduce a pyrazoloazole coupler into a light-sensitive material when a silver chlorobromide emulsion was used therein.
As a means for solving the above-described problem, the method of properly controlling pH of the emulsion coat is disclosed in Japanese Patent Application No. 162874/85. Although this method is very effective in severing the above-described interaction, it contains the following insufficiency. Even if the interaction inside the emulsion coat as it is can be cut off by previously controlling pH of the emulsion coat, the emulsion coat must be submitted to color development processing in order to make the emulsion coat develop a color. In the color development, it is, in general, to soak the light-sensitive material in a developer exhibiting pH ranging from about 9 to about 12 and, thereby, the pH of the emulsion coat increases sharply, and the interaction is newly generated in the processing solution. This interaction also is strong in case of silver chlorobromide.
It is important at any rate to cut off the actions of pyrazoloazoles upon silver halides, particularly silver chlorobromide, and it is expected to bring about improvements upon methods for cutting off the interaction between silver halides and pyrazoloazoles.